# std::common_type

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Defined in header `<type_traits>` |  |
template< class... T > struct common_type; |  | (since C++11)

Determines the common type among all types `T...`, that is the type all `T...` can be implicitly converted to. If such a type exists (as determined according to the rules below), the member `type` names that type. Otherwise, there is no member `type`.

- If sizeof...(T) is zero, there is no member `type`.
- If sizeof...(T) is one (i.e., `T...` contains only one type `T0`), the member `type` names the same type as std::common_type<T0, T0>::type if it exists; otherwise there is no member `type`.
- If sizeof...(T) is two (i.e., `T...` contains exactly two types `T1` and `T2`),

  - If applying std::decay to at least one of `T1` and `T2` produces a different type, the member `type` names the same type as std::common_type<std::decay<T1>::type, std::decay<T2>::type>::type, if it exists; if not, there is no member `type`;
  - Otherwise, if there is a user specialization for std::common_type<T1, T2>, that specialization is used;
  - Otherwise, if std::decay<decltype(false ? std::declval<T1>() : std::declval<T2>())>::type is a valid type, the member `type` denotes that type, see the conditional operator;

Otherwise, if std::decay<decltype(false ? std::declval<CR1>() : std::declval<CR2>())>::type is a valid type, where `CR1` and `CR2` are const std::remove_reference_t<T1>& and const std::remove_reference_t<T2>& respectively, the member `type` denotes that type; | (since C++20)

  - Otherwise, there is no member `type`.

- If sizeof...(T) is greater than two (i.e., `T...` consists of the types `T1, T2, R...`), then if std::common_type<T1, T2>::type exists, the member `type` denotes std::common_type<typename std::common_type<T1, T2>::type, R...>::type if such a type exists. In all other cases, there is no member `type`.

If any type in the parameter pack `T` is not a complete type, (possibly cv-qualified) void, or an array of unknown bound, the behavior is undefined.

If an instantiation of a template above depends, directly or indirectly, on an incomplete type, and that instantiation could yield a different result if that type were hypothetically completed, the behavior is undefined.

### Nested types

Name | Definition
`type` | the common type for all `T`

### Helper types

template< class... T > using common_type_t = typename common_type<T...>::type; |  | (since C++14)

### Specializations

Users may specialize `common_type` for types `T1` and `T2` if

- At least one of `T1` and `T2` depends on a user-defined type, and
- std::decay is an identity transformation for both `T1` and `T2`.

If such a specialization has a member named `type`, it must be a public and unambiguous member that names a cv-unqualified non-reference type to which both `T1` and `T2` are explicitly convertible. Additionally, std::common_type<T1, T2>::type and std::common_type<T2, T1>::type must denote the same type.

A program that adds `common_type` specializations in violation of these rules has undefined behavior.

Note that the behavior of a program that adds a specialization to any other template (except for `std::basic_common_reference`)(since C++20) from `<type_traits>` is undefined.

The following specializations are already provided by the standard library:

std::common_type<std::chrono::duration> (C++11) | specializes the std::common_type trait (class template specialization)
std::common_type<std::chrono::time_point> (C++11) | specializes the std::common_type trait (class template specialization)
std::common_type<std::pair> (C++23) | determines the common type of two `pair`s (class template specialization)
std::common_type<tuple-like> (C++23) | determines the common type of a `tuple` and a `tuple-like` type (class template specialization)
std::common_type<std::basic_const_iterator> (C++23) | determines the common type of an iterator and an adapted `basic_const_iterator` type (class template specialization)

### Possible implementation

    // primary template (used for zero types)
    template<class...>
    struct common_type {};

    // one type
    template<class T>
    struct common_type<T> : common_type<T, T> {};

    namespace detail
    {
        template<class...>
        using void_t = void;

        template<class T1, class T2>
        using conditional_result_t = decltype(false ? std::declval<T1>() : std::declval<T2>());

        template<class, class, class = void>
        struct decay_conditional_result {};
        template<class T1, class T2>
        struct decay_conditional_result<T1, T2, void_t<conditional_result_t<T1, T2>>>
            : std::decay<conditional_result_t<T1, T2>> {};

        template<class T1, class T2, class = void>
        struct common_type_2_impl : decay_conditional_result<const T1&, const T2&> {};

        // C++11 implementation:
        // template<class, class, class = void>
        // struct common_type_2_impl {};

        template<class T1, class T2>
        struct common_type_2_impl<T1, T2, void_t<conditional_result_t<T1, T2>>>
            : decay_conditional_result<T1, T2> {};
    }

    // two types
    template<class T1, class T2>
    struct common_type<T1, T2>
        : std::conditional<std::is_same<T1, typename std::decay<T1>::type>::value &&
                           std::is_same<T2, typename std::decay<T2>::type>::value,
                           detail::common_type_2_impl<T1, T2>,
                           common_type<typename std::decay<T1>::type,
                                       typename std::decay<T2>::type>>::type {};

    // 3+ types
    namespace detail
    {
        template<class AlwaysVoid, class T1, class T2, class... R>
        struct common_type_multi_impl {};
        template<class T1, class T2, class...R>
        struct common_type_multi_impl<void_t<typename common_type<T1, T2>::type>, T1, T2, R...>
            : common_type<typename common_type<T1, T2>::type, R...> {};
    }

    template<class T1, class T2, class... R>
    struct common_type<T1, T2, R...>
        : detail::common_type_multi_impl<void, T1, T2, R...> {};

### Notes

For arithmetic types not subject to promotion, the common type may be viewed as the type of the (possibly mixed-mode) arithmetic expression such as T0() + T1() + ... + Tn().

### Examples

Demonstrates mixed-mode arithmetic on a program-defined class:

Run this code

    #include <iostream>
    #include <type_traits>

    template<class T>
    struct Number { T n; };

    template<class T, class U>
    constexpr Number<std::common_type_t<T, U>>
        operator+(const Number<T>& lhs, const Number<U>& rhs)
    {
        return {lhs.n + rhs.n};
    }

    void describe(const char* expr, const Number<int>& x)
    {
        std::cout << expr << "  is  Number<int>{" << x.n << "}\n";
    }

    void describe(const char* expr, const Number<double>& x)
    {
        std::cout << expr << "  is  Number<double>{" << x.n << "}\n";
    }

    int main()
    {
        Number<int> i1 = {1}, i2 = {2};
        Number<double> d1 = {2.3}, d2 = {3.5};
        describe("i1 + i2", i1 + i2);
        describe("i1 + d2", i1 + d2);
        describe("d1 + i2", d1 + i2);
        describe("d1 + d2", d1 + d2);
    }

Output:

    i1 + i2  is  Number<int>{3}
    i1 + d2  is  Number<double>{4.5}
    d1 + i2  is  Number<double>{4.3}
    d1 + d2  is  Number<double>{5.8}

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2141 | C++11 | the result type of the conditional operator was not decayed | decayed the result type
LWG 2408 | C++11 | `common_type` was not SFINAE-friendly | made SFINAE-friendly
LWG 2460 | C++11 | `common_type` specializations were nearly impossible to write | reduced the number of specializations needed

### See also

common_with (C++20) | specifies that two types share a common type (concept)

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